The present invention relates to a method and apparatus for manufacturing superalloy disks to be particularly utilized for rotating body members of aircraft or power generation gas turbine engines in which highly improved fuel consumption efficiency is desired.
In conventional disc material technology, a nickel (Ni) base superalloy manufactured by forging a cast ingot is utilized for a turbine disk of an aircraft or a power generation gas turbine engine. However, in compliance with the recent technological requirements for highly improved performance of gas turbine engines with improved thermal efficiency, increased speed and reduced weight, it has been imperative to increase a volume ratio of .gamma.' precipitation phase in a structure of a turbine disk material. The tendency of this increase of the volume ratio has resulted in the increase of deformation resistance of turbine disk materials at high temperatures, a reduction in the forgeability of ingots, and an increase of segregation. For these and other reasons, it has become extremely difficult to forge and form a turbine disk having a complicated shape.
K. Iwai et al. in "Mechanical Properties of Ni-base Superalloy Disks Produced by Powder Metallurgy" (R-D KOBE STEEL ENGINEERING REPORTS, Vol. 37, No. 3, 1987, pp. 11-14) teaches a good example of a technique for solving the difficulty described above involving a near net shape working method by an isothermal forging means. This working method, as shown in FIG. 8, comprises the steps of producing fine powders from a molten material of a predetermined alloy composition by a gas atomizing method utilizing an Ar gas (step 40), and forming a billet by a hot extrusion or hot isostatic pressuring (hereinafter called "HIP") application (steps 41 to 44) so that solidified fine powders should exhibit a superplastic characteristic during a forging cycle at a low rate such as a strain of 2.times.10.sup.-4 /sec. (step 45). The temperatures of the billet and a mold are maintained at a constant temperature such as 1100.degree. C. in the forging cycle so as to obtain a product having a near net shape. Finally, heat treatment is carried out (step 46).
However, the conventional isothermal forging method described above involves the following defects or drawbacks.
(1) Low Productivity
The described method utilizes the isothermal forging method characterized by low rate of strain working as a method for improving the deformability, so that the working time is extremely long, resulting in low productivity. A lubricant for the mold is exposed at high temperature conditions for such a long time that the mold is extremely degraded.
(2) Too Many Manufacturing Steps
It is necessary to make the material into fine powders for minimizing segregation of elements and enabling the isothermal deformation, and therefore, the powder canning step (41) and the HIP or hot extrusion preforming step (42 or 43) are required. The need for these additional steps, of course, gives rises to additional equipment costs.
(3) Difficulty in Quality Control
Severe control is required for preventing the powder surface and a surface from the oxidation of foreign substances from intruding into the casing, which requires much labor for securing the reliability of the method.
(4) High Manufacturing Cost
The prolonged processing time in the HIP process in the third step 42 and the isothermal forging process in the fifth step 45 requires much energy, which results in the lowering of the productivity, the increase of the equipment cost and the increase of the maintenance cost, and therefore the increase of the manufacturing cost.